Particle accelerator producing a uniformly expanded particle beam of uniform cross-sectioned density

ABSTRACT

A particle accelerator useful for ion implantation using a high density ion source is provided with a means for increasing the cross-sectional area of the beam, a means for modifying the cross-sectional density of the beam to achieve a uniform particle density at the target area and a means for modifying particle trajectories and target holder so that the particles strike the substrate with a directed angle within 4* of parallel when they strike the plane of the target area.

United States Patent [19] Allison, Jr.

[451 Oct. 29, 1974 PARTICLE ACCELERATOR PRODUCING A UNIFORMLY EXPANDED PARTICLE BEAM OF UNIFORM CROSS-SECTIONED DENSITY Inventor: Robert William Allison, Jr.,

Richmond, Calif.

Assignee: Texas Instruments Incorporated,

Dallas, Tex.

Filed: July 13, 1972 Appl. No.: 271,497

us. Cl 250/398, 259/400, 250/492 Int. Cl H0lj 37/00, GOln 23/00 Field of Search..... 250/495 R, 49.5 c, 49.5 D, 250/495 T, 49.5 TE; 313/63; 328/228, 229, 230

References Cited UNITED STATES PATENTS 12/1958 Nygard' 250/495 Marker 250/495 3,120,609 2/1964 Farrell 250/495 3,434,894 3/1969 Gale 250/495 X 3,547,074 12/1970 Hirschfeld 250/495 X 3,621,327 11/1971 Hashmi 250/495 3,676,693 7/1972 Guernet 250/495 Primary Examiner-William F. Lindquist Attorney, Agent, or Firm-Harold Levine; James T. Comfort; Richard L. Donaldson [5 7 ABSTRACT 14 Claims, 12 Drawing Figures 'Pmmeum 29 m4 233845.312 sum 2 or 4 PARTICLE ACCELERATOR PRODUCING A UNIFORMLY EXPANDED PARTICLE BEAM OF UNIFORM CROSS-SECTIONED DENSITY BACKGROUND OF THE INVENTION This invention relates to particle accelerators and in particular to accelerators having a large beam crosssection.

It has been the practice of the prior art for particle accelerators to have particle beams which are narrow and focused so that when they reach the target, the beam cross-section is small in order to obtain a high particle density at the point of impact. When it is desired to cover a larger area with particles, various devices'such as electrostatic or electromagnetic deflectors were used to manipulate the beam over the desired area according to a predetermined scanning pattern.

Especially in the field of ion implantation of semiconductor devices, this technique affords a method of directing a beam of ions to a particular area of the substrate target material. The ion beams, in such cases, are maneuvered or programmed to scan a particular area of the substrate material, such as a silicon wafer, with, for example, either phosphorous or boron ions or other elements. Because of the fact that the density of the ion beams of the prior art devices is limited by the present state of the art in both intensity and density, there are a limited number of ions which can be transported and implanted during a given time period and, therefore, the time required to make a large area semi-conductor device may take many hours.

It is important in the ion implantation process that the density of ions striking the target be uniform over the target area. With the prior art devices, precautions must be taken in programming the scanning voltages to maintain a constant voltage during implantation thus requiring additional control equipment with respect to voltages and currents for the ion source and accelerating system.

It is also important in the ion implantation process that the ions strike the target material so that channeling is minimized, that is, avoiding deep penetration of ions when the incident ray beam enters the substrate material normal to a relatively open major plane and passes deep into the crystal. The prior art apparatus, to accomplish this would require additional deflection devices or extremely long beam paths.

SUMMARY OF THE INVENTION The apparatus of the present invention fulfills all the necessary criteria for ion implantation and further, decreases the process time for implanting a semiconductor device by including in its combination an ion source having a high ion intensity output beam and further comprising apparatus for enlarging the crosssectional area of the ion beam, modifying the crossbeam particle density is uniform over the target area.

It is yet another object of the present invention to provide a broad beam particle accelerator in which the particles all impinge on the target so that channeling is minimized.

It is a further object of the present invention to provide a particle accelerator having a high intensity parti- 'cle beam.

It is another object of the present invention to provide a particle accelerator having a high intensity particle beam of uniform cross-section.

Other and more particular objects of the present invention will be manifest upon a study of the following detailed description when taken together with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of the apparatus for obtaining the broad ion beam.

FIG. 2 is a cross-sectional view of the ion source apparatus.

FIG. 2A is a cross-sectional view of the charging section of the ion source apparatus.

FIG. 2B is an illustration of the disk used to maintain the discharge of the ion source.

FIG. 3 is a partial sectional view of the analyzer magnet.

FIG. 4 is a vertical cross-sectional elevation of the non-linear lens.

FIG. 5 is a horizontal cross-sectional view of the nonlinear lens taken at line 5-5 from FIG. 4.

FIG. 6 is a schematic block diagram of a typical ion implantation apparatus control system.

FIG. 7 is a vertical cross-sectional elevation of an electrostatic non-linear lens.

FIG. 8 is a horizontal cross-sectional view of the nonlinear lens of FIG. 7 taken at line 8-8.

FIG. 9 is a cross-sectional elevational view of a magnetic beam spreader.

FIG. 10 is an elevational view of another embodiment bf the target plate area.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1, there is shown a simplified schematic cross-sectional elevation of the essential elements of the ion implantation apparatus of the present invention which comprises, basically, a target 10, upon which are secured the devices into which it is desired to implant ions of a particular element, for example, wafers of silicon into which ions of phosphorous are to be implanted, an ion source 11 used to initially generate ions of the material being implanted, ion extractor 12, used to remove the ions from ion source 11 in the form of an ion beam 14 and providing them with an initial velocity, analyzer magnet apparatus 16 used for eliminating unwanted ions from ion beam 14, an accelerator section 21 used to achieve final acceleration of the ions and to contol beam divergence, a non-linear lens 18 used to adjust the ion beam cross-sectional density to be uniform over the target area, and a beam rotating solenoid 19 used to change the trajectory of the ions in beam 14 so that they strike target 10 at an angle which is within 4 of parallel when it strikes the plane of the target area. 7

Target is also arranged to be tilted so that, if desired, the directed angle of the beam with respect to the normal to the substrate is 7, plus or minus 2, as the desired angle or it can be any other convenient angle as may be found most beneficial for implantation and to avoid channeling.

Target 10 is also provided with a beam monitor plate immediately below the plane of the target area so the beam density and uniformity of cross-sectional ion density can be monitored at any time for quality control.

Ion source 11 is shown in greater detail in FIG. 2 in cross-sectional elevation and comprises, basically, an oven section 22 in which the material to be implanted is initially heated to cause vaporization and a discharge section 23 in which the vaporized material is contained as an ion plasma 24 (FIG. 2A). Ion source magnet 25 is used for containing the ion plasma in discharge section 23.

Oven section 22 comprises a chamber 28 having an orifice 29 facing discharge section 23 into which chamber 28 the material being implanted is placed, and source chamber holder 31 for holding source chamber 28 and which also contains ion source heater 32, which, for the embodiment shown, is an electrical resistance heater. Ion source chamber holder 31 is attached to apparatus housing34 by means of bolts or the like Discharge section 23 comprises, basically, a generally cylindrically shaped ion source assembly 43 enclosed by anode heating element 44 which, for the embodiment shown, is an electrical resistance heater which is used to prevent condensation of metallic vapor in the ion source.

As shown in greater detail in FIG. 2A, ion source assembly 43 comprises a generally cylindrically shaped ion anode 36 whose longitudinal axis 37 is coincident with that of ion oven section 22 (FIG. 2) and held in place by retainer rings 38 and 39 and'is electrically insulated therefrom by insulator rings 41 and 42.

A back plate 33 having an opening 40 aligned with axis 37 is attached to retainer ring 38 while a front plate 35 having an opening 52 also aligned with axis 37 is attached to retainer ring 39.

The entire ion source assembly 43 is assembled by stacking the back plate, retainer rings, anode and front plate together with shims of a brazing material sandwiched between and then heating the entire assembly to brazing termperature to achieve a solidly brazed assembly.

Two cathode disks 46a and 46b are shown in greater detail in FIG. 2B and further comprise several openings 55 cut in the outer edge to allow the vaporized material from oven section 22 to reach discharge section 23 and to leave discharge section 23 through front opening 52 and be accelerated and formed into ion beam 14.

Provision is also made for an electrical connection 45 to anode 36 so that it can be raised to an electrical potential relative to the rest of the apparatus.

Ion source magnet 25 comprises a solenoid, shown in FIG. 2 in partial section, whose axis of rotation and polar axis is coincident with the longitudinal axis of anode 36 and whose width is nearly that of ion discharge section 23.

Ion extractor 12 comprises an extractor port 47, the axis of which is coincident with that of anode 36 and an extractor electrode 48 defining a cup having an entrance port 49, a cup body 50 and an exit lip 51.

Extractor I2 is electrically insulated from ion source 11 so that it can be raised to any desired electrical potential in order to extract ions from ion source 11- and accelerate them to a particular initial velocity and shape them into an ion beam 14.

Analysis magnet apparatus 16, toward which ion beam 14 is initially accelerated, comprises, as seen in FIG. 3, a powerful magnet whose lines of force pass perpendicular to the axis of beam 14, in the present drawing, perpendicular to the surface of the drawing of FIG. 3, causing the ions in beam 14 to be deflected in an arc whose radius is dependent upon the velocity of the ion and its mass. In addition, the magnet edges are tilted with respect to the beam to provide vertical focusing. In effect, analyzer magnet 16 acts as a mass spectrometer which removes those ions which are heavier or lighter than the mass of the desired implanting material in order to obtain a beam of ions within a narrow mass range, and which focuses the resultant pure ion beam at or near the output edge of the magnet.

Analyzer magnet 16 apparatus also comprises an exit port 54 and, as will be noted below, the shape of the magnetic field at exit port 54 can also be controlled to achieve beam spreading, that is, to act as a means for creating a divergent beam of ions.

Under certain circumstances, where the initial ion velocity obtained by extractor 12 is insufficient, an accelerator section 21 is provided proximate exit port 54 of analyzer magnet 16 (FIG; 1) before beam 14 enters non-linear lens 18. The column gradient is adjusted so that the beam expands initially inside the column in a drift space.

As shown in FIG. 1, accelerator section 2lcomprises an upper accelerating electrode 27 and a lower accelerating electrode 30 for adjusting the beam energy. The gradient field of the accelerator section causes beam 14 to expand. The polar axis of the electrical field'is coincident with the axis of beam 14. 1

Expansion of the beam is illustrated in FIG. 1 by taking five typical particle trajectories 67, 68, 69, 67 and 68' and tracing their paths through the system after they leave exit port 54.

With reference to FIGS. 4 and 5, there are shown a vertical sectional view (FIG. 4) of non-linear lens 18 and a horizontal section taken through lens 18 at line 5-5 (FIG. 5).

Non-linear lens 18 comprises a generally cylindrical housing and pole support 58, whose axis of rotation is coincident with the axis of beam l4, a plurality of equally spaced poles or coil supports 59 attached to the inside of housing 58 and an equal plurality of coils 60 mounted on said poles whose polar axes are perpendicular to the axis of beam 14 such that a magnetic field is created having lines of force 62 (dotted lines) as shown in FIG. 5.

Another embodiment of an electrostatic non-linear lens 18' which can also be utilized is shown in FIGS. 7

and 8 and comprises a generally cylindrical housing and pole support 88 whose axis of rotation is coincident with the axis of beam 14, and a plurality of equally spaced poles or plates 89 attached to the inside of housing 88 by rods 90 which are electrically insulated from housing 89 by insulators 91, Rods 90 also act as conductors connecting poles 89to a power supply (not shown). I I

The polarity of the electrical charge placed on plates 89 is arranged to be opposite for adjacent poles in order to create the electrostatic lines of force as shown by dotted lines 92 in FIG. 8 which are also perpendicular to the axis of beam 14.

The ions, after passing through non-linear lens 18, since they are stilldivergent, must be changed as to their trajectory so that they strike target 10 with an angular spread which prevents channeling. Where necessary, as in the case of implantation with boron-silicon, the target is tilted with respect to the beam approximately 7 degrees.

Orientation of the wafer or target material with respect to the particle trajectory in order to prevent channeling can be achieved either by the use of 'a curved target holder plate 94 which exposes the wafers to the diverging particle beam, as shown in FIG. 10, or by the use of a flat target plate 10 and a beam rotator solenoid 19, as shown in FIG. I, which changes the trajectory of the beam particles so that they strike the target material within the desired angular range.

Beam rotator solenoid 19 is provided to perform an adjustment in the beam directed angle so that all trajectories are nearly parallel and comprises a solenoid coil 64 (FIG. 1) and an iron ring 65 whose axes of rotation and polar axes are coincident with the axis of beam 14 and which acts as a thin lens upon the beam. In addition, solenoid 19 provides focusing control of the beam at low energies.

Typical ion trajectories are shown in FIG; 1 by dashed lines 67, 68, 69, 67' and 68' In certain applications, an alternate method of causing the beam to diverge may be desired, rather than the use of electrostatic section 21. In such an instance, a magnetic means for causing beam divergence may be used as shown in FIG. 9.

In FIG. 9, magnetic beam spreader 17 is used in place of section 21 and comprises a solenoid 96 and an iron shim 97 whose axes of rotation and polar axes are coincident with the axis of beam 14.

With reference to FIG. 6, there is shown a schematic block diagram of a typical ion implantation apparatus control system of the present invention.

Basically, the system comprises, in addition to the basic elements of a target 10, an ion source 11, an ion extractor 12, an analyzer magnet 16, an accelerator section 21, a non-linear lens 18, a beam rotator l9 and a beam monitor 20, the additional items which energize and control the above elements, in particular, ion source power supply and control 72 which energizes oven heating element 32 and anode'heating element 44 and also provides the electrical potential for anode 36 and power for magnet 25. Also included in the system are ion extractor power supply and control 73, analyzer magnet power supply and control 74, accelerator section power supply and control 75, non-linear lens power supply and control 77, rotator solenoid power supply and control 78, monitor plate amplifier 79 and ion vacuum pump power supply and control 80, all of which are connected to implantation master control panel 81.

In addition, a main shell primary high voltage supply 82 is connected between master control panel .81 and extractor power supply and control 73 and ion source supply and control 72, while an analyzer primary high voltage supply 83 is connected between analyzer magnet power supply and control 74 and master control panel 81.

OPERATION To operate the ion implantation apparatus of the present invention, the items to receive the implanting material, such as silicon wafers, are placed at the target area 10 with appropriate masks placed over the chip or wafer to block out the areas where implantation is not desired. g

The system is then evacuated by energizing mechanical vacuum pump 85 and diffusion or ion vacuum pump 86.

After the system has been evacuated, the ion implantation material, such as phosphorous, is heated in source oven section 22 and the dissociated vapor enters ion source assembly 43 of discharge section 23 where it is ionized to create a charged plasma 24 within anode 36 which is raised to an electrical potential of the order of 3 kilovolts energized by the power supply of ion source power supply and control 72.

The intensity of the magnetic field created by ion source magnet 25 is adjusted through control 72 from master control panel 81 to provide containment of the plasma by anode 36.

The charged plasma in ion anode 36 is then drawn out by adjusting the electrical potential on extractor electrode 48 to achieve an accelerating voltage difference V(0) between ion source assembly and extractor electrode 48 which accelerates the ions to an initial velocity or energy level to define ion beam 14.

The magnetic field of analyzer magnet 16 is adjusted through its control 74 from master control panel 81 so that ions of the desired mass pass through analyzer magnet exit port 54.

The electrical potential of accelerator section 21 is adjusted through its control 75 to change the energy level and velocity of the ions as required for the particular implantation requirements.

The electrical potential on upper electrode 27 and lower electrode 30 of accelerator section 21 is adjusted through accelerator power supply and control 75 to provide the desired final implant energy.

The electrostatic field of accelerator section 21 is used to shape beam 14 as indicated by typical ion trajectories 67, 68, 69, 67' and 68'.

The intensity of the magnetic field of non-linear lens 18 is adjusted through its control 77 to achieve, as detected by beam monitor plate 20, a beam cross-section of uniform ion density.

The intensity of the magnetic field of beam rotator solenoid 19 is adjusted through its control 78 to achieve a perpendicular impingement of ions on target 10 which can also be detected by monitor plate 20.

Once adjusted, the silicon wafers at target area 10 can then be exposed to the beam of ions for the particular period of time necessaryto implant sufficient ions to create the particular semiconductor device desired;

It must also be noted that, although a magnetic and electrostatic means are disclosed causingthe beam to become divergent, to create uniformity of beam density at the target area and change the trajectory of ions so l claim:

1. A broad beam particle accelerator comprising:

source means for producing a beam of charged particles having a substantially uniform cross-sectional distribution of charged particles,

target area means for supporting at least one target in stationary relationship with said beam,

separator means for directing particles produced by said source means towards said target area and for separating particles from said beam to produce a beam of charged particles having a selected mass range,

means for substantially uniformly expanding the cross-sectional area of said beam having a selected mass range to produce a beam of larger area, and

means for causing said larger area beam to have a substantially uniform particle density over the cross-sectional area thereof at said target area.

2. The broad beam particle accelerator as claimed in 1 further comprising v means for changing the trajectories of the particles of said larger area beam of substantially uniform particle density so that their trajectories are generally parallel.

3. The broad beam particle accelerator as claimed in claim 2 wherein said means for changing the trajectory of said charged particles comprises a beam rotator solenoid having its polar axis coincident with the axis of said beam.

4. The broad beam particle accelerator as claimed in claim 2 further comprising means'for supplying an electrical current and controlling the voltage thereof connected to said means for changing the trajectory of said charged particles.

5. The broad beam particle accelerator as claimed in claim 1 wherein said source means for producing a beam of charged particles further comprises means for extracting said charged particles to form said beam having an approximately uniform crosssectional distribution of charged particles.

6. The broad beam particle accelerator as claimed in claim 1 wherein said means for expanding said beam comprises means for creating an electrical field defining an accelerating section having its polar axis coincident with the axis of said beam.

7. The broad beam particle accelerator as claimed in claim 6 wherein said means for creatingan electrical field defining an accelerating section comprises a first electrode having an entrance therein adapted to permit said beam of charged particles having a selected mass range to pass through,

a second electrode spaced from said first electrode toward said target area along the axis of said beam, and means for applying a voltage to said first and second electrodes and creating an electrical field for accelerating and expanding said beam.

8. The broad beam particle accelerator as claimed in claim 1 wherein said means for expanding said beam comprises a solenoid having its polar axis coincident with the axis of said beam, and

means for energizing said solenoid.

9. The broad beam particle accelerator as claimed in claim 1, wherein said means for expanding said beam to have a uniform particle density at the target area comprises a plurality of means disposed around the axis of said beam for producing a magnetic field surrounding said beam of charged particles, said means for producing a magnetic field being disposed with their polar axes perpendicular to the axis of said beam.

10. The broad beam particle acceleratoras claimed in claim 9 wherein said means for producing a magnetic field are disposed with adjacent'means arranged in opposite polarity.

11. The broad beam particle accelerator as claimed in claim 1 wherein said means for causing said beam to have a uniform particle density at the target area comprises a plurality of means disposed around the axis of said beam for producing an electrostatic field surrounding said beam of charged particles. 12. The broad beam particle accelerator as claimed in claim 11. wherein said plurality of means for producing an electrostatic field are disposed with adjacent means having opposite polarity.

13. The broad beam particle accelerator as claimed in claim 1 further comprising means for supplying an electrical potential and controlling the voltage thereof connected to said source means for producing charged particles, means for supplying an electrical current and controlling the voltage thereof connected to said means for expanding said beam, and means for supplying an electrical current and controlling the voltage thereof connected to said means for causing said beam to have a uniform particle density.' 14. The broad beam particle accelerator as claimed 50 in claim l'further comprising means for supplying an electrical current and controlling the voltage thereof connected to said separator means. 

1. A broad beam particle accelerator comprising: source means for producing a beam of charged particles having a substantially uniform cross-sectional distribution of charged particles, target area means for supporting at least one target in stationary relationship with said beam, separator means for directing particles produced by said source means towards said target area and for separating particles from said beam to produce a beam of charged particles having a selected mass range, means for substantially uniformly expanding the cross-sectional area of said beam having a selected mass range to produce a beam of larger area, and means for causing said larger area beam to have a substantially uniform particle density over the cross-sectional area thereof at said target area.
 2. The broad beam particle accelerator as claimed in 1 further comprising means for changing the trajectories of the particles of said larger area beam of substantially uniform particle density so that their trajectories are generally parallel.
 3. The broad beam particle accelerator as claimed in claim 2 wherein said means for changing the trajectory of said charged particles comprises a beam rotator solenoid having its polar axis coincident with the axis of said beam.
 4. The broad beam particle accelerator as claimed in claim 2 further comprising means for supplying an electrical current and controlling the voltage thereof connected to said means for changing the trajectory of said charged particles.
 5. The broad beam particle accelerator as claimed in claim 1 wherein said source means for producing a beam of charged particles further comprises means for extracting said charged particles to form said beam having an approximately uniform cross-sectional distribution of charged particles.
 6. The broad beam particle accelerator as claimed in claim 1 wherein said means for expanding said beam comprises means for creating an electrical field defining an accelerating section having its polar axis coincident with the axis of said beam.
 7. The broad beam particle accelerator as claimed in claim 6 wherein said means for creating an electrical field defining an accelerating section comprises a first electrode having an entrance therein adapted to permit said beam of charged particles having a selected mass range to pass through, a second electrode spaced from said first electrode toward said target area along the axis of said beam, and means for applying a voltage to said first and second electrodes and creating an electrical field for accelerating and expanding said beam.
 8. The broad beam particle accelerator as claimed in claim 1 wherein said means for expanding said beam comprises a solenoid having its polar axis coincident with the axis of said beam, and means for energizing said solenoid.
 9. The broad beam particle accelerator as claimed in claim 1, wherein said means for expanding said beam to have a uniform particle density at the target area comprises a plurality of means disposed around the axis of said beam for producing a magnetic field surrounding said beam of charged particles, said means for producing a magnetic field being disposed with their polar axes perpendicular to the axis of said beam.
 10. The broad beam particle accelerator as claimed in claim 9 wherein said means for producing a magnetic field are disposed with adjacent means arranged in opposite polarity.
 11. The broad beam particle accelerator as claimed in claim 1 wherein said means for causing said beam to have a uniform particle density at the target area comprises a plurality of means disposed around the axis of said beam for producing an electrostatic field surrounding said beam of charged particles.
 12. The broad beam particle accelerator as claimed in claim 11 wherein said plurality of means for producing an electrostatic field are disposed with adjacent means having opposite polarity.
 13. The broad beam particle accelerator as claimed in claim 1 further comprising means for supplying an electrical potential and controlling the voltage thereof connected to said source means for producing charged particles, means for supplying an electrical current and controlling the voltage thereof connected to said means for expanding said beam, and means for supplying an electrical current and controlling the voltage thereof connected to said means for causing said beam to have a uniform particle density.
 14. The broad beam particle accelerator as claimed in claim 1 further comprising means for supplying an electrical current and controlling the voltage thereof connected to said separator means. 